1. Field of the Invention
The subject matter of the present invention involves methods, software products, and systems that determine three dimensional velocity of an object. More particularly, the methods, software products, and systems determine radial and transverse velocities of the object.
2. Statement of the Problem
Some systems that measure the velocity of moving objects use the Doppler effect, where a change of frequency occurs depending on the motion of the object toward or away from the observer. The Doppler effect occurs in various forms of waves including sound, light, and electromagnetic waves. Doppler radar was developed during World War II to detect aircraft and other moving objects. Doppler radar has also been used in meteorological applications to measure precipitation and wind. In one meteorological application, mono-static Doppler radar measures the radial velocity of wind.
Other meteorological applications use Doppler radar to measure the velocity of wind. One example of a meteorological application is a multiple Doppler radar system, where two or more radar are directed towards a region from different directions. The Doppler radar systems then obtain the velocity of the wind from different directions and generate a two dimensional or three dimensional wind field. Unfortunately, using multiple Doppler radar systems is expensive. Also, synchronization between the multiple Doppler radar systems is difficult to achieve.
Another example is a bi-static radar network that includes one transmitter and at least two receivers placed at different angles. The multiple receivers obtain the velocity of wind from each of the receiver directions. Problems of the bi-static radar network include sidelobe, varying sample volume, and limited spatial coverage.
Another example is a Doppler beam swing (DBS) wind profiler. The DBS wind profiler points the radar beam in different directions and obtains the velocity in corresponding directions. One problem is the DBS wind profiler is only applicable to highly homogenous wind so that the different sample volume has the same wind. Unfortunately, wind is not necessarily homogenous.
Another method for determining velocity is the interferometry technique. The interferometry technique determines the transverse velocity from the interference effect of the scattered wave since the interference pattern moves with the scatterer""s motion. In one example, the interferometry technique is understood as a change in correlation. If the sample volume follows the motion of the distributed scatterers, the correlation is maximum. Otherwise, the correlation of the scattered wave signal decreases. If the sample volume is in the opposite direction of the motion of the target distributed scatterers, the correlation is even less.
One example of the interferometry method is the spaced antenna (SA) system. In the SA system, multiple receivers are co-located with the transmitter. The SA system then processes the received signals to determine the cross-correlation function and the wind velocity. One prior system by Liu et al in 1990 relates the complex cross correlation to the wind and refraction index xcex94n statistics. Another prior system by Doviak et al in 1996 derives the cross-correlation function and extends it to a large class of xcex94n statistics.
One prior system by the National Center for Atmospheric Research is the Multiple Antenna Profiler (MAPR). The MAPR system includes 4 vertically pointing sub-arrays for reception and uses SA techniques to measure wind. The MAPR measures the motion of atmospheric echoes as the scatterers in the atmosphere move over the radar.
One processing method for obtaining the transverse wind is the full correlation analysis by Briggs et al in 1984. Another processing method is the intersection method by Doviak in 1994 and 1995. The intersection method estimates both auto-and cross-correlation functions. Another processing method is the cross/auto-correlation by Doviak et al in 1997. Another processing method is the slope at zero lag method by Lataitis et al in 1995. The slope at zero lag method is difficult to obtain high accuracy in the case of time lag interval comparable to the time shift of the correlation peak. One problem with these methods is the effect of system noise on auto-correlation.
The invention solves the above problems by determining a three dimensional velocity of an object wherein the three dimensional velocity comprises a radial velocity and a transverse velocity. A velocity determination system receives a first signal from the object. The velocity determination system then receives a second signal from the object. The velocity determination system then correlates the first signal and the second signal. The velocity determination system determines the radial velocity based on a ratio of the correlation of the first signal and the second signal. The velocity determination system then determines a transverse velocity based on a ratio of the correlation of the first signal and the second signal in a forward mode and a backward mode along a transverse direction.
In one embodiment, the velocity determination system transmits a third signal towards the object. In some embodiments, the object comprises wind. In some embodiments, the first signal and the second signal comprise electromagnetic waves, light waves, or sound waves. In one embodiment, the velocity determination system determines a cross correlation coefficient for the first signal and the second signal. The velocity determination system then determines a ratio of the cross correlation coefficient for the first signal and the second signal.
In other embodiments, the velocity determination system combines the first signal end the second signal. In one embodiment, the velocity determination system receives one other signal from the object and correlates the combination of the first signal and the second signal with the others signal. The velocity determination system then determines the radial velocity based on a ratio of the combination of the first signal and the second signal with the other signal and determines a transverse velocity based on ratio of the correlation of the combination of the first signal and the second signal with the other signal in the forward mode and the backward mode along the transverse direction.